Electric amplifying circuits



Aug. 30, 1932. a. D. BEDFORD ELECTRIC AMPLIFYlNG' C IRGUITS Filed Sept. 26, 1930 5 as a; I,

' i. IO 4' 52 3 -M @QQU N 0 Grid Potential Inv enb OT.

Buvnice D. Bedfovd,

His Abbowne Patented Aug. 30, 1932 UNITED STATES PATENT OFFICE BURNICE D. IBEDFORD, OF SCHENECTADY, N EW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK ELECTRIC AMPIJIYING CIRCUITS Application filed September 26, 1930. Serial No. 484,627.

My invention relates to electric circuits for amplifying electric signals and more particularly to electric circuits including controlled electric valves for accurately reproducing and amplifying electric signals.

Heretofore there have been proposed various arrangements for amplifyin electric signals such as are common in te ephony, radiotelephony, the transmission of pictures,

etc, by means of grid controlled electric valves, the grids of which are energized in response to the signal to be amplified. These arrangements of the prior art are, however, adapted for use only in connection with electrio valves of the pure electron discharge type in which the current in a valve is continuously controlled by the potential on its grid. Such arrangements have the inherent disadvantages of large tube losses and low power output for ordinary operating voltages.

It is an object of my invention to provide an electric circuit for amplifying electric signals which shall overcome the disadvantages 95 of the arrangements heretofore proposed and will be efficient, and reliable and which shall be capable of delivering a large power output.

It is a further object of my invention to provide a circuit for amplifying electric signals which shall be adaptable for use either in connection with electric valves of the pure electron discharge type or with vapor electric discharge valves in which the starting of current in a valve is controlled by the potential impressed upon its control grid but in which the current in the valve may be interrupted only by reducing the anode potential below its critical value.

It is a further object of my invention to provide an improved excitation for electric valves used in electric amplifying circuits.

In accordance With my invention, I provide an electric circuit, including electric Valves, for convertin direct current to alternating current and I provide high frequency periodic potential of a particular wave form for exciting the grids of the valves. This high frequency grid potential is modulated by the signal to be amplified. The output of such a circuit will have a component of the same frequency as the high frequency grid excitatlon and another component of the same frequency as that of the signal to be amplified. By means of a suitable filter the high frequency component is eliminated leaving only that component having the frequency of the signal to be amplified.

For a better understanding of my inventlon, together with other and further objects thereof, reference is had to the accompanying drawing and its scope will be pointed out in the appended claims. Fig. 1 of the drawing is a diagrammatic representation of my invention as applied to an electric power converting apparatus of the type known in the art as a parallel inverter, by means of which an electric signal may be amplified; Fig. 2 comprises reproductions of oscillograms of certain operating characteristics of the apparatus illustrated in Fig. 1, and Figs. 3 and 4 show certain characteristics of one feature of my invention by means of which I secure the desired excitation for the grids of the electric valves.

Referring more particularly to Fig. 1 of the accompanying drawing, I have illustrated an electric power converting apparatus for amplifying an electric signal impressed upon the circuit 10 and delivering the amplified output to the circuit 11. This power con verting apparatus derives energy from the direct current circuit 12 and includes an inductive winding 13 provided with an electrical midpoint connected to the positive direct current terminal through a smoothing reactor 14 and a resistor 15. The inductive winding 13 may be a reactor or, as shown, the primary winding of a transformer 16, the secondary winding 17 of which is connected to the output circuit 11. Between opposite terminals of the inductive winding 13 and the negative direct current terminal 12 are connected electric valves 18 and 19. These valves may be of any of the several types well known in the art but I prefer to use vapor electric discharge valves provided with an anode, a cathode and a control grid. A capacitor 20 is connected between the circuits of the electric valves 18 and 19 for commutating the current between them. The grid circuits of the electric valves18 and 19 include opposite halves of the secondary winding of a grid transformer 22. The primary winding 23 of the transformer 22 includes the electric signal'to be amplified, which may be derived from the circuit 10 by means of a transformer 2e, and a source of relatively.

high frequency alternating potentional of substantially triangular waveform, shown as derived from the terminals of'thecapaci' tor 25. In some cases 1t may also. be desirable to nclude acurrent limiting resistor 26 and anegative bias battery 27. in the gridorder to filter from the output of the trans formeriG the component frequencies other thanthose appearing in the signal impressed upon the electric circuit 10, I have provided aiilter circuit comprising reactors 32. and 33 connected in series with the circuit 11 and a resistor 34; and capacitors 35 and 36 connected across said circuit. I

The above described apparatus relies for its'operationupon the fact that, while it is necessary for currentto flow continuously in either the electric valve 18 or 19, it is not essential that the time interval du'rin which the current fiows in these two valves should be equal. Currentin the electric Valves 18 and 19 corresponds to half cycles of the alternating current output of opposite polarity.

" by properly varying the ratio of the time during which current flows in the respective valves the average current-over a complete cycle maybe Varied from approximately zero to maximum in either direction.

The "operation of the electric power converting apparatus, which will be understood by those skilled in the art, is as follows: If

a positive. potential; is initially impressed upon the grid ofelectric valv-e'l8,'eurrent will flow from the positive direct'current line 12 through the left hand portion of the inductive winding 13, and the electric valve 18 to the other direct current line 12. As the cur rent builds up in the left handportion. of the inductive winding 13,'th1s'wind1ng acts as an auto-transformer and a still more positive voltage is induced in'the right hand portion. The full potentialacrossthe'terminals of" the winding '13, which is approximately twice that of the direct current circuit, is

impressed on the capacitor 20 which becomes charged to this potential. When the grid potential reverses its polarity, so that the grid of the valve 18 is made negative while that of the valve 19 is made positive, the capacitor 20 is short circuited through the electricvalves 18 and 19.-- The potential across-the capacitor 20 is oppositein direction to that tending to sendothe current through theielectricvalve18 and many times its magnitude with the result that the current in this valve is instantly interrupted and current'flows only in the valve 19. When the grid potentialagain reverses polarity, the current is transferred from electric valve 19 to"electric valve 18 and the cycle issrepeated indefinitely. The-reactor l l serves to main tain-tliecurrent taken fromthe direct current circuit 12'. substantially constant while the re sister 15 serves to 'li-mit the high frequency 1 component of current'delivered to the output circuit-.11 as willbe explained hereinafter. 1

in explaining the operation of the electric power converting apparatus as an amplifier,

itwill' be assumed that a high-frequency alternating potential of substantially triangle 'lar' wave form appears at the terminals of the capacitor '25 and the operation of the apparatusg'for generating this alternating. po-

tentialy-i'vill be described hereinafter; It will also be; assumed that-the signal to be amplifiedis a GO'cycle alternating potential having an amplitude somewhat less than that of the high frequency potential. Theresultant of these two-potentials, which is applied to the grid circuit of the valves 18 and 19, is shown by'Curve I of Fig. 2, in. which the line a represents the cathode potential of theelectric valves 18 and 19. Sincev the grid potentials of the valves 18 and 19 are opposite in polarityit is clear that the portions:of the Curve 1 abovethe line a may betaken as the positive potential applied to the grid of the electric valve 18 whilethat/p'ortion of the CurveI-below the lineia may be taken as the 19- As the grid signal. to be amplified varies from the maximum in one direction to" the maximum in the other direction, it will be seenthatthe relative intervals during which the valves. 18 and 19 areconducting vary between a maximum conducting period of. the valve 18 and a minimum of the valve 19, to 'a maximum conducting period of the valve 19 and minimum of the'v alve. 18. Curve II of Fig-211s a reproduction of an oscillogram of the current output of one of-the electric valves, for example, valve 18. The output curve of the valve 19 will be similar but displaced by 180 in'time phase and having an opposite polarity with respect to the load circuit'11 The resultant'output currentwill be an alternating current of a frequency the sameas that of the high frequency grid exciting potential, but in which the positive and positive potential applied to theelectric valve negative half cycles are of unequal value. Such a wave is substantially the equivalent of a symmetrical wave plus a direct current component of a polarity dependent upon whether the positive or negative half cycle is the larger. This direct current component reverses its polarity at a frequency equal to that of the signal to be amplified and, by filtering out the high frequency component, the resultant output of the apparatus will be a periodic current of a frequency and wave form similar to that of the electric signal to be amplified. Curve III of Fig. 2 is a reproduction of an oscillogram of the amplifier output with a cycle electric signal and approximately a 1.000 cycle grid exciting potential. It will be obvious to those skilled in the art that the accuracy with which the electric signals may be reproduced increases with an increase in frequency of the grid exciting potential. As described above, I have provided a filter circuit comprising the series connected reactors 32 and 33 and the parallel capacitors 35 and 36 for filtering out the high frequency component of the output current. It has been found that the resistor 15 connected in series with the direct current line 12 and a relatively high resistance 34 connected across the primary winding 17 of the transformer 16 will aid in eliminating the high frequency component from the output circuit 11. However, this filter circuit is merely illustrative and any other form of filter circuit examples of which are well known in the art may be substituted in place thereof without departing from my invention.

' While any arrangement for producing a high frequency alternating potential of substantially triangular wave shape may be used to supply the grid excitation of electric valves 18 and 19, I prefer to use the apparatus de scribed above. In the operation of this apparatus the alternating current generator 28,

which may be either of the dynamo-electric or the oscillation generator type, delivers a high frequency potential of substantially sine wave form to the grid circuit of the electric valve 29 which is preferably of the pure electron discharge type having a grid potentialanode current characteristic similar to that shown in Curve IV of Fig. 3. The amplitude of the alternating potential of the generator 28 is sufficient to considerably excite the valve 29 beyond saturation, that is, such as to operate the valve 29 at both extremes of the grid potential-anode current characteristic as shown by Curve V of Fig. 3. When the valve 29. is operating at either extreme of this char- 60 acteristic curve it will be seen that a rela by Curve VI of Fig. 3 and Curve VII of Fig.

4. Curve VIII of Fig. 4 represents the grid potential of the valve 29. The anode current of electric valve 29 is caused to flow through the capacitor 25 either by connecting it directly in series in the anode circuit or by inserting a series transformer 31 in order to step up the voltage applied to the capacitor 25. The voltage in abvolts appearing across a capacitor is represented by the equation e= idt C where c=the capacity in farads; i=current in abamperes; t=time in seconds, so that, during those portions of the cycle of the anode current in which the current is constant, which would be for a complete half cycle if the anode current were exactly rectangular in wave form, the voltage across the capacitor 25 will build up at a constant rate and, when the anode current reverses its polarity, the capacitor 25 will likewise build up to an opposite polarity at a constant rate. The potential across the capacitor 25 is represented byCurve IX of 4:, which, it is seen, is substantially triangular. The approximation of this curve to an exactly triangular wave is directly proportional to the approximation of the anode current to a rectangular wave which may be made very close by applying a very large grid potential to the electric valve 29. 7

While I have shown and described what I at present consider the preferred embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. Apparatus for amplifying an electric signal comprising a source of direct current and a load circuit, an interconnection between said source and said circuit including an inductive winding and an electric valve, means independent of the signal for periodically rendering said valve conducting, and means responsive to the signal to be amplified for determining the interval during which the valve remains conducting.

2. Apparatus for amplifying an electric signal comprising a source of direct current, a load circuit, an interconnection between said source and said circuit including an inductive winding and a pair of electric valves, means independent of the signal for periodically and alternately rendering said valves conducting, and means responsive to the signal to be amplified for determining the periods during which the valves remain conducting.

3. Apparatus for amplifying an electric signal comprising asource of direct current,

a load circuit, an interconnection between said source and said circuit including anoinductive winding and a pair of electric valves, means for alternately initiating :a flow of current in'said valves, means responsive to the fiow'of current in one of said valves for interrupting the current in the other, and means responsive to the signal to be amplified for determining the time intervals between' the starting of current insuccessive valves.

5. Apparatus for. ampllfylng an electrlc signal comprising a source of. direct current,

a load circuit, an interconnection between said source and said circuit including an inductive winding and a pair of electric valves, means for alternately initiating a current in said-valves, means responsive to the starting' of current in one of said valves for interrupting the current in the other, and me ans responsive to the instantaneous value of the signal to be amplified for determining the difference in the time intervals between the starting of current in-the successive valves.

6. Apparatus for amplifying an electric signal comprising a source of direct current, a load circuit, an interconnection between said source and said circuit including an inductive winding and a pair of electric valves, means for alternately starting a, current in said valves, means responsive to. the startingof current in one of said valves for interrupting the current in the other, andmeans for causing the difference in the time intervals beeach provided with a control element, means for energizing said control elements with a perlodic potential of a frequency relatively high with respect to that of the signal and of an amplitude less-than that of the signal,

and means for modulating said high frequencypotential with said signal.

8. Apparatus forv amplifying an electric- 7 signal comprlsing a source of direct current,

a load circuit, an interconnection between sa1d source and said circuit including an inductive winding and apa r of electric valves,

- 9. "Apparatus for amplifying an electricsignal comprising a source of direct current, a load circuit,- an interconnection between said source and said circuit comprising an inductive winding,the electrical midpoint of V which is connected to one direct current terminal, a pairofelectric valves, provided with 7 control grids connected between opposite terminals ofsaid inductive winding and the other direct current terminal, and a capacitor connected between the circuits of said valves for commutatingthe current between them, a grid transformer, grid circuits for each of said valves includin opposite halves of the secondary winding of said transformer, an energizing circuit for the primary winding of said transformer including said electric signal and a source of alternating potential of substantially triangular wave form, of a frequency relatively high with respect to that of said signal and of lesser amplitude,and means connected in said outputcircuit for filtering substantially all frequencies except those of the signal.

10. The method of operating an electric valve, provided with a control element, for amplifying an electric signal, which comprises generating a periodic potential of substantially triangular wave form and of a relatively high frequency with respect to that of the signal and of a lesser amplitude, combining the signal with the high frequency potential, and applying the resultant to the control. element of theelectric valve.

11. The method of amplifying an electric signal which comprises generating a periodic potential of substantially triangular wave formand of a relatively high frequency with respect to that of the signal and'of a lesser amplitude,combiningthe signal with the high frequencypotential, controlling the average value of a periodic current in accordance with the diiference in thetime intervals of successive positive and negative half cycles of said combined potential wave with respect to a fixed potential, and filtering from said periodic current substantially all frequencies except thoseincluded in the signal to be amplified. t r

12. The method of'aniplifying an electric signal which comprises generating a periodic potential of substantially triangular wave form and of a relatively high frequencywith respect to'that'of the signal and of a lesser amplitude, combining the signal with the highfrequency potential, controlling the flow of. current from a direct current source by means of saidcombined potential to produce 

